1. Field of the Invention
The present invention relates to a system and method for suppressing the formation of oxygen inclusions and surface blisters in glass sheets and the resulting glass sheets.
2. Description of Related Art
Liquid crystal displays (LCDs) are flat panel display devices that include flat glass substrates or sheets. The fusion process is a preferred technique for producing sheets of glass used in LCDs because the fusion process produces sheets whose surfaces have superior flatness and smoothness compared to sheets produced by other methods. The fusion process is described in U.S. Pat. Nos. 3,338,696 and 3,682,609, the contents of which are incorporated herein by reference.
Many of the glass sheets manufactured for flat panel display applications, particularly those formed by the fusion process (also referred to as the downdraw process or the slot draw process), are melted and formed with components made from refractory metals, e.g. platinum or platinum alloys. This is particularly true in the fining and conditioning sections of the fusion process, where refractory metals are employed to minimize the creation of gaseous inclusions within the glass sheets. To further minimize the creation of gaseous inclusions within the glass sheets, the fusion process often employs arsenic as a fining agent. Arsenic is among the highest temperature fining agents known, and, when added to the molten glass bath, it allows for O2 release from the glass melt at high melting temperatures (e.g., above 1450° C.). This high temperature O2 release, which aids in the removal of bubbles during the melting and fining stages of glass production, coupled with a strong tendency for O2 absorption at lower conditioning temperatures (which aids in the collapse of any residual gaseous inclusions in the glass), results in a glass sheet that is essentially free of gaseous inclusions. From an environmental point of view, it would be desirable to provide alternative methods of making such high melting point and strain point glass sheets without having to employ arsenic as a fining agent.
It would also be desirable to find alternative methods for making such glass sheets via the downdraw process in which the glass sheets have very little if any gaseous inclusions or surface blistering. One such method is described in U.S. Pat. No. 5,785,726 which discloses a humidity controlled enclosure that surrounds all or a portion of a platinum-containing vessel and is used to control the dew point outside the vessel in order to reduce the formation of gaseous inclusions in glass sheets. Another method for reducing the formation of gaseous inclusions in glass sheets is described in U.S. Pat. Nos. 6,128,924 and 5,824,127 which disclose the use of various batch constituents to minimize the water content in the glass composition and thus the hydrogen concentration on the inside surface of the platinum-containing vessel. Although the methods disclosed in the patents mentioned above successfully reduce the formation of gaseous inclusions in glass sheets formed in systems utilizing platinum-containing vessels, it would be desirable to provide alternative methods to prevent the formation of gaseous inclusions in glass sheets.